X-ray Computed Tomography (“CT”) is a technique that noninvasively generates cross-sectional images of the linear attenuation coefficients (“LACs”) of materials in an object of interest. X-ray CT has been used extensively in medical, industrial, and security applications, such as generating cutaway images of a brain, detecting flaws in a piston, or observing internals of baggage at an airport. The LAC is a measure of the distance-dependent attenuation of X-rays as they pass through a certain material and has units of inverse length (e.g., per centimeter). Typically, X-ray CT systems employ an X-ray source, which emits a pencil, fan, or cone beam of photons through an object with an initial intensity. A single element, linear, or area array X-ray detector measures the final intensities of the X-ray beams that pass through the object and impinge on the detector pixels. For X-ray CT, the source and detector are positioned at various angles relative to the object and measurements of the final intensity at each angle are collected. The set of measurements for each angle is referred to as a projection. Various techniques may be used to collect measurements at different angles relative to the object, for example, the source and detector may be stationary and the object may be moved, the object may be stationary and the source and detector may be moved, and multiple stationary sources and detectors may be positioned at different angles. CT algorithms then reconstruct, from the collection of measurements, a 3D image of the object that specifies the LAC for each volume element (“voxel”) within the volume of the object. Cross-sectional images are generated from the 3D image.
There is strong interest in the security field in efforts to extend CT technology to help characterize physical and chemical characteristics of objects being scanned. For example, for airport security, it would be helpful to determine whether an object found in checked baggage contains an explosive material. While X-rays are ill-suited for determining specific molecular structure and composition, X-ray CT technology can be used to generate a high-resolution estimate of both the electron density (ρe) and effective atomic number (Zeff) of compounds in each voxel within an object. Conventional CT employs a single source that generates X-rays over a certain spectral energy distruibution. The two independent parameters ρe and Zeff, however, cannot be determined from a single-spectrum measurement, such as the measurements of a single-energy-source CT scanner. So, Dual-Energy CT (“DECT”) has been used to enable these estimates.
DECT generates measurements for two photon spectral energy distributions. Because there are two sets of partially orthogonal measurements, the values for ρe and Zeff can be estimated. DECT typically employs either voltage switching or a sandwich detector. With voltage switching, the X-ray source voltage and source filtration are modulated to different levels to generate low- and high-energy photon spectral energy distributions. With a sandwich detector, a filtering material is placed between two detector elements in the same X-ray beam line. The first detector in the beam sees an initial photon spectral energy distribution with a lower energy bias relative to the spectral distribution observed at the second detector due to attenuation by the additional filtration. DECT generates a low-energy LAC volume specifying the LAC (μlow) of the low-energy spectrum in each voxel, and a high-energy LAC volume specifying the LAC (μhigh) of the high-energy spectrum in each voxel. The electron density (ρe) is roughly proportional to μhigh, and the effective atomic number (Zeff) can be approximated as a function of the ratio of μlow over μhigh. These approximations for calculating (ρe) and (Zeff demonstrate poor accuracy. In addition, these approximations do not compensate for differences in spectral responses between scanners or spectral drift due to wear in a single scanner.